1. Field of the Invention
The invention relates generally to voltage regulators and specifically to limiting the short circuit current in a voltage regulation circuit.
2. The Prior Art
FIG. 1 is a schematic illustrating a prior art voltage regulator circuit. Circuit 10 includes a power-controlling pass device, for example PMOS transistor 15, coupled between supply voltage 20 and output node 25. A stable output voltage Vout over a defined current IL range is produced between output node 25 and ground. The output of amplifier 30 is coupled to the gate of transistor 15, therefore regulating the behavior of transistor 15. Reference resistors 35 and 40 produce a voltage divider input for amplifier 30 and complete a regulation loop created by transistor 15, amplifier 30, and resistors 35 and 40. Capacitor 45 compensates the regulation loop.
Amplifier 30 compares the voltage across resistor 40 with reference voltage Vbg. Output voltage Vout is determined by the combination of reference voltage Vbg and resistors 35 and 40. As current IL increases above its maximum level, amplifier 30 starts to work in a non-liner mode (i.e. saturation) and as a consequence there is a decline the output voltage Vout. The voltage versus current behavior depends on the characteristics of transistor 15. One problem with circuit 10 is that if transistor 10 is large (for example, in order to have good power supply rejection ratio), then amplifier 30 saturates for high values of current IL in a regulator that features low current load range. This means that the regulator presents a very high short circuit current compared to the typical regulator load current. Such short circuit current primarily depends on characteristics of transistor 15 and is not directly controllable.
One solution for the above referenced problem features a switch connected between the gate of transistor 15 and the supply voltage 20, and controlled by the load current value IL. When the current IL is lower than a predetermined threshold the switch is open and the regulator works in normal operation. When IL is higher than the threshold, the switch is closed thus fixing the voltage at the controlling node of transistor 15, and so limiting the short circuit current of the regulator at the selected current threshold. The problem with this approach is that the rapid on-off state sequencing of the switch causes oscillation in circuit behavior.
What is needed is a current limitation circuit based on a simple architecture that provides a predictable output response and does not alter the behavior of the regulator in normal operation.